Recovery of aromatic hydrocarbons from mixtures of hydrocarbons by selective extraction with a substituted piperazine solvent

ABSTRACT

WHERE X DENOTES OR ACETYL AND R DENOTES LOWER ALKYL.   1-X,4-R-PIPERAZINE   RECOVERY OF AROMATIC HYDROCARBONS FROM MIXTURES OF HYDROCARBONS WHICH CONTAIN PARAFFINS AND POSSIBLY OLEFINS AND/OR NAPTHENES BY USING AS EXTRACTANTS PIPERAZINE DERIVATIVES HAVING THE GENERAL FORMULA:

United States Patent Oifi-ce 3,580,839 Patented May 25, 1971 US. Cl. 208-326 8 Claims ABSTRACT OF THE DISCLOSURE Recovery of aromatic hydrocarbons from mixtures of hydrocarbons which contain paraflins and possibly olefins and/ or naphthenes by using as extractants piperazine derivatives having the general formula:

CH2CH2 Where X denotes formyl or acetyl and R denotes lower alkyl.

This invention relates to an improved process for the recovery of aromatic hydrocarbons from mixtures of hydrocarbons by selective extraction with piperazines.

It is known that hydrocarbon mixtures consisting of paraflinic and aromatic constituents can be separated into a raflinate high in parafiins and a rafiinate high in aromatics by means of solvents. Examples of suitable solvents are gamma-butyrolactone, N-alkylpyrrolidones and also N-hydroxyalkylpyrrolidones, oxazolidones, morpholine and its derivatives, tetramethylsulfone alkane dinitriles, propylene carbonate, dicarboxylic amides, diethylene glycols and triethylene glycols.

The aim is that the extract should become enriched with aromatics as much as possible. Another desirable property of a suitable solvent is resistance to hydrolytic influences under the conditions of the extraction. This is particularly the case when water is added to the solvent. Moreover the solvent used should have the lowest possible viscosity. Solvents hitherto known have however not fulfilled these conditions.

The hydrocarbon feeds usually employed in industry contain not only aromatics and paraffins but often considerable amounts of olefins and/or naphthenes, so that .the solvent for extraction of the aromatics and for their recovery in great purity has to have a high selectivity, i.e. has to have not only a high solvent power for aromatics but also a high separating power for olefins and napththenes. In many cases, owing to inadequate selectivity of the extractant, it is necessary to carry out a selective hydrogenation of the hydrocarbon feed to satisfy the unsaturated compounds prior to the separation of the aromatics. This step involves additional expenditure.

It is also already known from British patent specification No. 1,057,390 that N-substituted piperazines may be used as extractants. The said piperazines have the disadvantage however that they do not combine all the desirable properties such as selectivity, resistance to hydrolysis, low viscosity and appropriate boiling range.

The object of this invention is to provide a process which ensures high selectivity. Another object of the invention is to provide a process in which efiicient separation of aromatic hydrocarbons and olefins is achieved.

In accordance with this invention these and other objects and advantages are achieved in an improved process for the recovery of aromatic hydrocarbons from mixtures of hydrocarbons which contain paraffins by treatment with piperazines by known extraction methods, wherein the improvement consists in using as the extractant a piperazine derivative having the general formula:

where X denotes a formyl or acetyl radical and R denotes a lower alkyl radical.

Preferred aromatic hydrocarbons which can be recovered by the process according to this invention are mononuclear and dinuclear aromatic hydrocarbons having six-membered aromatic rings which may bear aliphatic hydrocarbon radicals having one to four carbon atoms as substituents. The process has proved to be particularly suitable for the recovery of mononuclear aromatic hydrocarbons which have up to three alkyl groups having one to three carbon atoms. The recovery of benzene, toluene and xylenes has acquired particular industrial importance. Examples of suitable aromatic hydrocarbons are benzene, toluene, o-xylene, p-xylene, ethylbenzene, isopropylbenzene, p-diisopropylbenzene and styrene.

As feedstocks containing the said aromatic hydrocarbons there are suitable crude oil cuts such as straight-run gasolines, reformed gasolines, diesel oils, gas oils, lubricating oils or hydrocarbon mixtures obtainable by known petrochemical methods such as cracked gasolines, pyrolysis gasolines or low temperature coking gasolines, which may contain, in addition to paraffin hydrocarbons. olefins or naphthenes, for example in amounts of up to 50% by Weight preferably up to 20% by weight. Feedstocks which contain 3 to 60% by weight of aromatic hydrocarbons are particularly suitable.

The extractants used are piperazine derivatives having the general formula:

where X denotes a formyl or acetyl radical and R denotes a lower alkyl radical. Piperazines having the above formula where X denotes a formyl radical and R denotes an alkyl radical having one to four carbon atoms, particularly one or two carbon atoms, have proved to be particuuarly suitable. Examples of suitable starting materials are: N formyl-N'-methylpiperazine, N-formyl-N-ethylpiperazine, N formyl N'-isopropylpiperazine, N-acetyl-N'- methylpiperazine, N acetyl N'-ethylpiperazines and N acetyl N butylpiperazine. N-formyl-N'-methylpiperazine has acquired particular industrial importance.

The piperazine derivatives may be used in pure form or as mixtures with one another or as mixtures with other extractants such as tetramethylsulfone, diethylene glycol or triethylene glycol. The mixtures used as extractants preferably contain more than 60%, particularly more than 80%, by weight of the said piperazines with reference to the whole mixture. The addition of ethylene glycols, particularly with from 10 to 40% by weight of ethylene glycol with reference to the piperazine used, has proved to be especially advantageous. It is advantageous to add water to the piperazines or to the mixtures containing piperazines. It is preferable to add to 25%, particularly 5 to 15%, by weight of water with reference to the extractant or mixture of extractants used.

The process may be carried out at atmospheric pressure or at superatmospheric pressure, for example up to 5 atmospheres. It is however also possible to use subatmospheric pressure. The temperature used is generally from 0 C. up to the boiling point of the solvent. As a rule e.g. the liquid-liquid extraction is carried out at room temperature or slightly elevated temperature, particularly at from 30 to 60 C.

All known methods for separating hydrocarbons by means of selective solvents may be used for recovering aromatic hydrocarbons by the process according to this invention. Use may be made not only of liquid-liquid extraction or countercurrent distribution but also the method of extractive distillation or countercurrent scrubbing. It may also be advantageous to combine two or more of these methods together. In order to obtain pure aromatics it is advantageous in the case of a liquid-liquid extraction to use a back-flow for example of aromatics. Since aromatics and the piperazine are as a rule miscible without limit it is advantageous to add to the extractant such an amount of water that a miscibility gap occurs. It may also be advantageous to use a back-flow of another compound, for example a hydrocarbon having a suitable boiling point. Water or another solvent may however also be added to the separating apparatus and the backfiow produced within the column itself. It is advantageous to use aliphatic hydrocarbons such as propane, butane, pentane or isooctane as solubilizers.

The invention is illustrated by the following example. The parts given in the following example are by weight. They bear the same relation to parts by volume as the kilogram to the liter.

EXAMPLE A mixture consisting of 50 parts of n-hexane, 60 parts of cyclohexane, parts of cyclohexane, 10 parts of cyclohexane and 30 parts of benzene is fed at the rate of 200 parts per hour into the middle of a sieve plate column having a length of 3 meters. 1500 parts per hour of N- formyl-N'-methylpiperazine containing 10% of water is fed into the top of the column and 400 parts Per hour of isooctane is fed into the bottom of the column. To improve the extraction effect, the column of liquid is set in pulsating movement. The extraction temperature is 40 C. A raflinate containing less than 1% by weight of benzene is taken ofi at the top of the column. The extract withdrawn in the lower part of the column has the following 4 composition after isooctane and the extractant have been separated:

Percent Benzene 99.8 Cyclohexane 0.1 Cyclohexane 0.1 Hexane 0.05

where X denotes a formyl radical or an acetyl radical and R denotes a lower alkyl radical.

2. A process as claimed in claim 1 wherein a hydrocarbon mixture is used which contains from 3 to by weight of aromatic hydrocarbons.

3. A process as claimed in claim 1 wherein a hydrocarbon mixture is used which contains mononuclear aromatic hydrocarbons having one to three alkyl groups containing one to four carbon atoms.

4. A process as claimed in claim 1 wherein a liquidliquid extraction is used.

5. A process as claimed in claim 1 wherein a liquid liquid extraction is used at a temperature of from 30 to 60 C.

6. A process as claimed in claim 1 wherein a mixture of a piperazine derivative having the formula (I) and ethylene glycol which contains more than 60% by Weight of piperazine derivative is used as the extractant.

7. A process as claimed in claim 1 wherein the extractant cantains from 5 to 25% by weight of water.

8. A process as claimed in claim 1 wherein N-formyl- N-methylpiperazine is used as extractant.

References Cited UNITED STATES PATENTS 2,092,739 9/1937 Van Dijk 208289 2,437,357 3/1948 McKinnis 260674 2,670,810 3/1954 Dorsey 260674 HERBERT LEVINE, Primary Examiner US. Cl. X.R. 260674 mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 580,839 Dated May 25 1971 Inventofls) Ernst Fuerst 8c Hans -Martin Weit z It is certified that: error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 10, Dec. 50, 1967 should read Dec. 19, 1967 Column 2, line 67, "N-acetyl-N'-ethylpiperazines" should read N-acetyl-N'-ethy1piperazine Signed and sealed this 2 th day of August 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attosting Officer WILLIAM E. SCHUYLER, JR. Commissioner of Patents 

